Somatosensory evoked potentials (SEPs) and cortical single unit responses elicited by mechanical tactile stimuli in awake monkeys.

The origins of surface recorded evoked potentials have been investigated by combining recordings of single unit responses and somatosensory evoked potentials (SEPs) from the postcentral gyrus of 4 alert macaque monkeys. Responses were elicited by mechanical tactile stimuli (airpuffs) which selectively activate rapidly adapting cutaneous mechanoreceptors, and permit patterned stimulation of a restricted area of skin. Epidurally recorded SEPs consisted of an early positive complex, beginning 8-10 msec after airpuff onset, with two prominent positive peaks (P15 and P25), succeeded by a large negative potential (N43) lasting 30 msec, and a late slow positivity (P70). SEPs, while consistent in wave form, varied slightly between monkeys. The amplitude of the early positive complex was enhanced by increasing the number of stimulated points, or by placing the airpuffs in the receptive fields of cortical neurons located beneath the SEP recording electrode. SEP amplitude was depressed when preceded 20-40 msec earlier by a conditioning stimulus to the same skin area. Single unit responses in areas 3b and 1 of primary somatosensory (SI) cortex consisted of a burst of impulses, beginning 11-12 msec after the airpuff onset, and lasting another 15-20 msec. Peak unitary activity occurred at 12-15 msec, corresponding to the P15 wave in the SEP. No peak in SI unit responses occurred in conjunction with the P25 wave. Although SI neurons fired at lower rates during P25, the lack of any peak in SI unit responses suggests that activity in other cortical areas, such as SII cortex, contributes to this wave. Most unit activity in SI cortex ceased by the onset of N43, and was replaced by a period of profound response depression, in which unit responses to additional tactile stimuli were reduced. We propose that the N43 wave reflects IPSPs in cortical neurons previously depolarized and excited by the airpuff stimulus. Late positive potentials (P70) in the SEP had no apparent counterpart in SI unit activity, suggesting generation at other cortical loci.